The landscape of RGsB is:
Nav from MK3 (MK2 i'm not sure)makes the Screen. If there is a tv-modul integrated, the tv modul forwards the nav signal. only when you turn to tv or rearcam, the tv modul generates/shows/overlays the screen.
you can check this, when you stay on nav screen in tv mode. the nav shows an empty screen.
When you replace the tv modul and use an usual tft incl. driverboard, you must handle the sources by yourself. most tft has a seperated av in, which can enabled by one wire for rearcam.